investor agreement

[roman]

I have an investor in my limited company who has already handed me a cheque in return for equity.
However he hasn't signed the investor agreement yet as he wants a parts changed (which I'm happy with).

He has paid me in advance as he will be away for a month, and wants to give me the neccassary capital in the meantime.

what can I give him as a legal reciept that would protect his interests (as well as mine)?
would a letter stating that the monies were in leiu of equity (without stating the amount of equity) according to any future investor agreement as agreed between the relevant parties suffice?

Are there any dangers in the above approach?

will this letter have to signed by me to make it binding?
would email correspondance suffice?
or would it have to be scanned first?

any pointers would be welcome